i have a similar problem with mine, but thats because of my monitor's cable.
luckily, my monitor isn't cheap and has a detatchable vga cable from the actual monitor, so once i get some money and can be bothered, i'll go and buy a new cable. i should really get one soon, as this shadow can sometimes bring on a headache, it cant be doing me much good.
i find having a little play with the pc connection end of my cable can alter the shadow, make it more or less, and after a 5 minute fiddle i can normally reduce the shadow so it's only really noticeable around large objects on the screen, you have to look quite close to see it on small text.
so you could try that,
and as fallout mentioned, some devices not magnetically sheilded can make your monitor go dodgy. i know someone who stupidly put his subwoofer right next to his monitor, the picture was jumping around the screen like a whore on a good day.
TMB - under construction
Sticking feathers up your butt doesn't make you a chicken.